The present invention pertains to a key member for use in inserting fuel rods into a nuclear fuel assembly grid or in removing the fuel rods from the nuclear fuel assembly grid, and a method for insertion or removal of fuel rods using the same key member. The invention further relates to a method for disassembling a nuclear fuel assembly using the same key member.
In a known nuclear fuel assembly, a pair of top and bottom nozzles are arranged in a spaced relation to each other, and a plurality of grids each formed of straps are disposed between the nozzles so as to be spaced from each other. Control-rod guide pipes and measuring instrument pipes are inserted through the grid cells of the grid, fixedly secured to the grid, and connected to the nozzles. A plurality of fuel rods are inserted through the grid cells of the grids and resiliently supported by springs formed on the grids.
In a conventional assembly method of the nuclear fuel assembly as constructed above, if the fuel rods are simply inserted into the grid cells, the fuel rods are constricted between with the springs and dimples on the grids, so that the outer peripheral surface of each fuel rod may be scratched. Various proposals have hitherto been made in order to circumvent this problem. For example, as disclosed in U.S. Pat. No. 5,068,081, a method utilizing a deflecting jig of an expander type capable of being enlarged in diameter has been proposed. In this method, an opening for inserting a key member is formed in each grid, and the springs are deflected by inserting the jig into the grid cells of the grid and enlarging the same. Then, the key member is inserted through the opening to maintain the springs deflected, and fuel rods are smoothly inserted through the grid cells.
However, in the above method, the deflecting jig must be used since the key member is not provided with the function to deflect the springs, and hence, in addition to the necessity of preparing the deflecting jig, an extra step of applying the deflecting jig is required.
Furthermore, when it is necessary to dismantle the nuclear fuel assembly, if the fuel rods can be removed from the grid by, for example, inserting the key member through the opening and operating the same to release the engagement of the fuel rods with the grids, the fuel rods thus removed can be put into repeated use for a new nuclear fuel assembly. However, once the assembling of the nuclear fuel assembly is completed, the opening for the insertion of the key member is obstructed by the control-rod guide pipes or measuring instrument pipes, so that the key member cannot be inserted any longer. Therefore, it has been desired to devise an arrangement to provide access to the space for the insertion of the key member.